In the defect inspection for transparent plate materials, it is necessary to distinguish defects, such as bubbles, foreign matters, or scratches, from pseudo defects that do not affect the quality of the transparent plate materials, such as dust and grime. Depending on the positions of defects (whether defects are located on a main surface, in the inside, or on a rear surface) in the transparent plate materials and the depths of defects when they are located in the inside, the required quality level is different in some cases. Therefore, it has been demanded not only to detect defects but also to identify the positions of the defects in the thickness direction of the transparent plate materials.
In addition to these requirements for inspection performance, requirements for industrial applications are also imposed on defect inspection methods. In such requirements, the inspection performance should not be reduced depending on the sizes of transparent plate materials, the methods should be able to be applied to transparent plate materials having various thicknesses, and the methods should be able to be applied to continuous forming processes (online) for transparent plate materials, which are typically represented by a float process for flat glass.
To distinguish between defects and pseudo defects, there is a method, which uses a transmission dark-field optical system having a rod-shaped light source and a light-blocking mask combined together therein as disclosed in JP-A-8-201313. This method uses a difference in light-scattering directivity between defects and pseudo defects to distinguish between the defects and the pseudo defects. However, since light-scattering directivity differs depending on the types of defects, and since pseudo defects also exhibit various light-scattering directivity, it is difficult to distinguish between defects and pseudo defects just by light-scattering directivity. In addition, since the method disclosed in JP-A-8-201313 uses a transmission optical system, it is difficult to identify the positions of defects in the thickness direction of the transparent plate materials.
To distinguish between defects and pseudo defects, there is another method (hereinafter called an edge light method), in which light is emitted toward the inside of a transparent plate material from its end face to detect scattered light caused by a defect as disclosed in JP-A-10-339705 and JP-A-11-264803. The light incident on the end face and coming into the inside of the transparent plate material advances through the inside of the plate material, repeating total reflection. The light is scattered at a portion where a defect exists, and exits from a main surface or a rear surface of the transparent plate material. The light advancing through the inside of the transparent plate material is not scattered by a pseudo defect attached to the main surface or the rear surface of the transparent plate material. Therefore, when cameras are disposed at a main surface side and a rear surface side of the transparent plate material, only defects can be detected. In addition, by comparing detection signals sent from the cameras disposed at the main surface side and the rear surface side of the transparent plate material, the position of a defect in the thickness direction of the transparent plate material can be determined to some extent.
Patent Document 1: JP-A-8-201313
Patent Document 2: JP-A-10-339705
Patent Document 3: JP-A-11-264803